CMS-2412 Implement delete site-template REST method in SiteTemplateResource

Added deleteSiteTemplate method to SiteTemplateResource, added unit tests, update DeleteSiteTemplate handler to return key of deleted template and thorw exception if template was not found
Merge pull request #445 from enonic/CMS-2412

CMS-2412 Implement delete site-template REST method in SiteTemplateResou...